.Net Core DI-相同类型的多个实例

Question

First of all, I saw some topics similar to mine, however I think they don't really answer my issue.

I am creating an application, which, depending on configuration can work in multiple modes. The app is a simple importer/exporter of data from database to files. It can actually transfer data:

• between files and DB
• between DB and another DB

The first case is not problematic, the second one is. My application has classes like:

• FilesReader
• FilesWriter
• DbReader
• DbWriter

I think you can see how I can connect them together to read from one source and write to another. Problem appears when I want to use DbReader and DbWriter at the same time. Since both of them require IDbClient in their constructors, I need to have a way to pass to them different instances of it (since DbReader will read from Database A and DbWriter will write to database B). In reality, I actually have more classes that need IDbClient . For example I have DbDataCache , which will need instance of IDbClient to connect with Database A, and so on.

There are a few ways that I think of how it could be achieved. However, I don't really like any of them. Here're my ideas:

1. While registering each class to the container, I could register them "manually" - create each IDbClient , create each class that depends on it and pass the right instances to them - I don't like it, because I lose the easiness of registration, it becomes quite complicated and there is a lot of new keyword use.

2. I could create a DbClientFactory and register IDbClient instances with some names (like DatabaseA and DatabaseB ) - I don't like it, because the classes that need IDbClient will now need to know the name that they should require. So, probably DbReader would need to have somethink like this in its constructor

_dbClient = _dbFactory.Get("DatabaseA");

while DbWriter will have:

_dbClient = _dbFactory.Get("DatabaseB");

What do you think? Maybe there is some other DI container that would be more suitable for my needs than the .Net Core built-in one?

Answer 1

As per my understanding from your explaination is that; Your solution would be to have new instance of IDbClient served on each request to DI Container. If that's true try to register your service as Transient

 services.AddTransient<IMyService, MyService>();

as per docs

Transient Lifetime

If in doubt, make it transient. That's really what it comes down to. Adding a transient service means that each time the service is requested, a new instance is created.

Answer 2

I would use the same approach that is used for loggers in .NET Core itself. So, instead of registering in DI each separate logger you usually register ILoggerFactory and then inject it in the necessary class and create a logger with CreateLogger(name) function.

So, the similar way as with logging you can register in DI some implementations of IDbClientFactory, inject it where it's needed and call something like dbClientFactory.CreateReader(dbName1) or dbClientFactory.CreateWriter(dbName2) to create actual reader or writers.

Funny thing. About 2 weeks ago we had published a .NET Core global tool which does the same tasks: importing and exporting the data from some common formats (JSON, XML) to DB and vice versa. The tool itself and the underlying library are totally open-source and published on GitHub . So, feel free to use that code in your own project if you find there something useful.